1. Field of the Invention
The present invention relates to trapping for preventing a white spot caused by plate misregistration.
2. Description of the Related Art
There is an image forming apparatus configured to print a full color image by overlapping images which are divided into a plurality of color plates, on a sheet. When misalignment (plate misregistration) is caused between the color plates in a case where images of the color plates are superimposed, a white spot appears on the boundary between image objects of different color plates, which brings the ground color of the sheet into sight. Trapping processing is performed in order to suppress the generation of the white spot. The trapping processing extends the region of the image object of one color plate so that the region of the image object of one color plate overlaps with the region of the image object of the other color plate.
Because the trapping processing causes the image objects to overlap with each other, the trapping processing can suppress the generation of the white spot even when the plate misregistration is caused. However, because the trapping processing causes the image object of one color plate to overlap with the image object of the other color plate, two colors are mixed in the edge portion of the image object, to cause a tint change. Therefore, it is important to suppress the visual influence caused by the tint change in the edge portion of the image object while preventing the white spot caused by the plate misregistration in the trapping processing.
Japanese Patent Application Laid-Open No. 2008-141623 discusses an image forming apparatus which decides whether to perform trapping processing for a color plate other than a black color plate (K plate) and performs the trapping processing. The image forming apparatus considers the concentration of a surrounding pixel of an object pixel of the trapping processing, and does not perform the trapping processing, for example, on a clearance sandwiched between two image objects to suppress the visual change on the boundary between the image objects.
FIG. 8A illustrates an image in which an image object (background) including a magenta color plate (M plate) is adjacent to an image object (character “F”) including an image of a K plate, and an image of a cyan color plate (C plate). A positional relationship between color plates in a region surrounded with dashed lines of FIG. 8A is illustrated in an arrangement pattern 801 of FIG. 8B. A dashed line extending in a longitudinal direction illustrated in FIG. 8B illustrates the boundary between an image object (character “F”) and an image object (background) when the image objects are ideally arranged.
The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2008-141623 performs the trapping processing on the C plate and the M plate other than the K plate in the image of the arrangement pattern 801 to produce the image of an arrangement pattern 802. In such a case, in the image of the arrangement pattern 802, a bluish color (the mixed color of the C plate and the M plate) which is not produced when the character “F” (the mixed color of the K plate and the C plate) and the background (the M plate) overlap with each other appears on the boundary between the character “F” and the background by performing the trapping processing. If the plate misregistration is caused when printing is performed based on the image of the arrangement pattern 802, the image of an arrangement pattern 803 may be printed. That is, the overlap region of the C plate and the M plate is enlarged, which causes a conspicuous bluish color (the mixed color of the C plate and the M plate).
When the trapping processing is performed on the K plate as on the C plate, the overlap region can be made smaller than the arrangement pattern 803. However, because the region extension of the K plate which is a conspicuous color causes a large visual change, the region extension is not desirable.